As far as I can tell, from my own knowledge and experience in Israel, the answer is more simple than most think: vandalism.
Israel got lots of teenagers who are damaging public property on purpose, wherever and whenever they can. The reasoning for them doing it is beyond the scope of this question and site, so I won’t go there.
Toilet seats are also one of their victims, they just break them on purpose. Of course that proper management will replace the seats, but in places where such teenagers visit often, it becomes an expense they just can’t afford, and since they can’t install security cameras or place guards in the toilets, it can’t really be stopped, so their only option is to leave the toilets without seats to break.